among cosmologists, but also skepticism.
Victor Mosquera for Quanta Magazine

A
surprise discovery announced a month ago

suggested that
the early universe looked very different

than previously
believed.

Initial theories
that

the discrepancy
was due to dark matter

have come under
fire.

The news about the
first stars in the universe always seemed a little off.

Last July, Rennan
Barkana, a cosmologist at Tel Aviv University, received an
email from one of his longtime collaborators, Judd Bowman.

Bowman leads a
small group of five astronomers who built and deployed a radio
telescope in remote western Australia. Its goal: to find the whisper
of the first stars. Bowman and his team had picked up a signal that
didn't quite make sense.

He asked Barkana to
help him think through what could possibly be going on.

For years, as radio
telescopes scanned the sky, astronomers have hoped to glimpse signs
of the first stars in the universe.

Those objects are
too faint and, at over 13 billion light-years away, too distant to
be picked up by ordinary telescopes. Instead, astronomers search for
the stars' effects on the surrounding gas.

Bowman's
instrument, like the others involved in the search, attempts to pick
out a particular dip in radio waves coming from the distant
universe.

The measurement is
exceedingly difficult to make, since the potential signal can get
swamped not only by the myriad radio sources of modern society - one
reason the experiment is deep in the Australian outback - but by
nearby cosmic sources such as our own Milky Way galaxy.

Still, after years
of methodical work, Bowman and his colleagues with the Experiment
to Detect the Global Epoch of Reionization Signature (EDGES)
concluded not only that they had found the first stars, but that
they had found evidence that the young cosmos was significantly
colder than anyone had thought.

The
EDGES result could be interpreted as a completely
new way that ordinary material might be interacting
with dark matter.

Barkana was
skeptical, however.

"On the one
hand, it looks like a very solid measurement," he said. "On the
other hand, it is something very surprising."

What could make the
early universe appear cold?

Barkana thought
through the possibilities and realized that it could be a
consequence of the presence of dark matter - the mysterious
substance that pervades the universe yet escapes every attempt to
understand what it is or how it works.

He found that the
EDGES result could be interpreted as a completely new way that
ordinary material might be interacting with dark matter.

Yet in the weeks
since the announcement, cosmologists around the world have expressed
a mix of excitement and skepticism.

Researchers who saw
the EDGES result for the first time when it appeared in Nature
have done their own analysis, showing that even if some kind of dark
matter is responsible, as Barkana suggested, no more than a small
fraction of it could be involved in producing the effect. (Barkana
himself has been involved in some of these studies.)

And experimental
astronomers have said that while they respect the EDGES team and the
careful work that they've done, such a measurement is too difficult
to trust entirely.

"If this
weren't a groundbreaking discovery, it would be a lot easier for
people to just believe the results," said Daniel Price, an
astronomer at Swinburne University of Technology in Australia
who works on similar experiments.

"Great claims
require great evidence."

This message has
echoed through the cosmology community since those Nature
papers appeared.

The Source of a Whisper

The day after
Bowman contacted Barkana to tell him about the surprising EDGES
signal, Barkana drove with his family to his in-laws' house.

During
the drive, he said, he contemplated this signal, telling his wife
about the interesting puzzle Bowman had handed him.

Bowman and the
EDGES team had been probing the neutral hydrogen gas that filled the
universe during the first few hundred million years after the Big
Bang.

This gas tended to absorb ambient light, leading to what
cosmologists poetically call the universe's "dark ages."

Although the cosmos
was filled with a diffuse ambient light from the cosmic microwave
background (CMB) - the so-called
afterglow of the Big Bang - this
neutral gas absorbed it at specific wavelengths. EDGES searched for
this absorption pattern.

As stars began to
turn on in the universe, their energy would have heated the gas.

Eventually the gas reached a high enough temperature that it no
longer absorbed CMB radiation. The absorption signal disappeared,
and the dark ages ended.

The absorption
signal as measured by EDGES contains an immense amount of
information. As the absorption pattern traveled across the expanding
universe, the signal stretched.

Astronomers can use
that stretch to infer how long the signal has been traveling, and
thus, when the first stars flicked on. In addition, the width of the
detected signal corresponds to the amount of time that the gas was
absorbing the CMB light.

And the intensity
of the signal - how much light was absorbed - relates to the
temperature of the gas and the amount of light that was floating
around at the time.

Many researchers
find this final characteristic the most intriguing.

"It's a much
stronger absorption than we had thought possible,"

...said
Steven Furlanetto, a cosmologist at the University of
California, Los Angeles, who has examined what the EDGES data would
mean for the formation of the earliest galaxies.

Lucy
Reading-Ikkanda
Quanta Magazine

Source: arXiv:1609.02312v3 Figure 1 (expected

doi:10.1038/nature25792 Figure 2 (observed)

The most obvious
explanation for such a strong signal is that the neutral gas was
colder than predicted, which would have allowed it to absorb even
more background radiation.

But how could the
universe have unexpectedly cooled?

"We're talking
about a period of time when stars are beginning to form,"
Barkana said - the darkness before the dawn. "So everything is
as cold as it can be.

The question is: What could be even
colder?"

As he parked at his
in-laws' house that July day, an idea came to him:

Millicharged dark matter could
interact with ordinary matter, but only very weakly.

Intergalactic gas
might then have cooled by,

"basically
dumping heat into the dark matter sector where you can't see it
anymore," Furlanetto explained.

Barkana wrote the
idea up and sent it off to Nature.

Then he began to
work through the idea in more detail with several colleagues. Others
did as well.

As soon as the
Nature papers appeared, several groups of theoretical
cosmologists started to compare the behavior of this unexpected type
of dark matter to what we know about the universe - the decades'
worth of CMB observations, data from supernova explosions, the
results of collisions at particle accelerators like the
Large Hadron Collider, and
astronomers' understanding of how
the Big Bang produced hydrogen,
helium and lithium during the universe's first few minutes.

If millicharged
dark matter was out there, did all these other observations make
sense?

They did not...

More
precisely, these researchers
found
that millicharged dark matter can only make up a small fraction of
the total dark matter in the universe - too small a fraction to
create the observed dip in the EDGES data.

"You cannot
have 100 percent of dark matter interacting," said
Anastasia Fialkov, an astrophysicist at Harvard University
and the first author of
a
paper submitted to Physical Review Letters.

Yet black holes
also produce other forms of radiation, like X-rays, that haven't
been seen in the early universe.

Because of this,
astronomers remain skeptical that black holes are the answer.

Is It Real?

Perhaps the
simplest explanation is that the data are just wrong...

The measurement is
incredibly difficult, after all. Yet by all accounts the EDGES team
took exceptional care to cross-check all their data - Price called
the experiment "exquisite" - which means that if there is a flaw in
the data, it will be exceptionally hard to find.

This antenna for EDGES

was deployed in 2015 at a remote
location

in western Australia where it would
experience

little radio interference.
LoCo Lab

The EDGES team
deployed their radio antenna in September 2015.

By December, they
were seeing a signal, said Raul Monsalve,
an experimental cosmologist at the University of Colorado, Boulder,
and a member of the EDGES team.

"We became
suspicious immediately, because it was stronger than expected."

And so they began
what became a marathon of due diligence.

They built a
similar antenna and installed it about 150 meters away from the
first one. They rotated the antennas to rule out environmental and
instrumental effects.

They used separate
calibration and analysis techniques.

"We made many,
many kinds of cuts and comparisons and cross-checks to try to
rule out the signal as coming from the environment or from some
other source," Monsalve said.

"We didn't
believe ourselves at the beginning. We thought it was very
suspicious for the signal to be this strong, and that's why we
took so long to publish."

They are convinced
that they're seeing a signal, and that the signal is unexpectedly
strong.

"I do believe
the result," Price said, but he emphasized that testing for
systematic errors in the data is still needed.

He mentioned one
area where the experiment could have overlooked a potential error:

Any antenna's
sensitivity varies depending on the frequency it's observing and
the direction from which a signal is coming.

Astronomers can
account for these imperfections by either measuring them or modeling
them.

Bowman and
colleagues chose to model them. Price suggests that the EDGES team
members instead find a way to measure them and then reanalyze their
signal with that measured effect taken into account.

The next step is
for a second radio detector to see this signal, which would imply
it's from the sky and not from the EDGES antenna or model.

Scientists with the
Large-Aperture Experiment to Detect the Dark Ages (LEDA)
project,
located in California's Owens Valley, are currently analyzing that
instrument's data.

Then researchers
will need to confirm that the signal is actually cosmological and
not produced by our own Milky Way. This is not a simple problem.

Our galaxy's radio
emission can be thousands of times stronger than cosmological
signals.

On the whole,
researchers regard both the EDGES measurement itself and its
interpretation with a healthy skepticism, as Barkana and many others
have put it.

Scientists should
be skeptical of a first-of-its-kind measurement - that's how they
ensure that the observation is sound, the analysis was completed
accurately, and the experiment wasn't in error.

This is,
ultimately, how science is supposed to work.

"We ask the
questions, we investigate, we exclude every wrong possibility,"
said
Tomer Volansky, a particle physicist at Tel Aviv University
who collaborated with Barkana on one of his follow-up analyses.

"We're after
the truth. If the truth is that it's not dark matter, then it's
not dark matter."